The present invention relates generally to network interfacing, and more particularly, to an apparatus and method for slicing a received modulated signal including quadrature amplitude modulated signal to recover transmitted data.
The transmission of various types of digital data between computers continues to grow in importance. A predominant method of transmitting such digital data includes coding the digital data and modulating a high frequency carrier signal in accordance with the coded digital data. A coding and modulation technique known as quadrature amplitude modulation provides for modulating both the carrier amplitude and the carrier phase to represent encoded data. The QAM modulated high frequency carrier signal is transmitted across a network physical transmission medium such as electrical cable, fiber optic, RF, or other medium to a remote computing station.
At the remote computing station, the high frequency carrier signal must be received, demodulated, and decoded, including slicing, to recover the original data. In the absence of any distortion across the network medium, the received signal would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated and decoded using known techniques to recover the original data.
One problem with networks is that the physical medium and network topology tend to distort the carrier signal, especially at high frequencies. Branch connections and different lengths of such branches cause reflections of the transmitted signal. Such problems are even more apparent in a network which uses home telephone wiring cables as the network physical medium. The typical wiring of the telephone network is designed for the xe2x80x9cplain old telephone servicexe2x80x9d (POTS) signals in the 3-10 kilohertz frequency and are not designed for transmission of high frequency carrier signals in a frequency range greater than 1 MHz. The high frequency carrier signal is also distorted by transients in wiring characteristics due to on-hook and off-hook switching and noise pulses of the POTS (e.g. ringing). The high frequency carrier is further distorted by spurious noise caused by electrical devices operating in close proximity to the xe2x80x9ccablexe2x80x9d medium.
Such distortion of frequency, amplitude, and phase of the high frequency carrier signal degrades network performance and tends to impede the design of higher data rate networks.
One technique for compensating for such distortion would be to slow the data rate by using a lower payload encoding. For example, in an 8 data bits/baud payload encoding system, the receiver must distinguish between 256 distinct constellation coordinates, each representing a distinct combination of a carrier signal phase and amplitude. Each constellation coordinate corresponds to 8 bits of data information and is typically called a symbol. Alternatively, in a 4 data bits/baud payload encoding system, the receiver only needs to distinguish between 16 distinct constellation coordinates and in a 2 bits/baud payload encoding system, the receiver only needs to distinguish between 4 distinct constellation coordinates. The 2 bits/baud system will be more tolerant to distortion because the distorted carrier phase and amplitude are less likely to mis-map to an incorrect constellation coordinate.
More specifically, referring to FIGS. 1a and 1b, a known 2 bits/baud constellation 20 (2 bits per symbol) and a 4 bits/baud constellation 22 (4 bits per symbol) are shown respectively. Constellation 20 includes 4 defined constellation coordinates 24(a)-24(d). Each constellation coordinate 24(a)-24(d) represents a data symbolxe2x80x94a unique combination of carrier magnitude and phase. For example, the magnitude of vector 26 and the phase angle 28 correspond to constellation coordinate 24(b).
Constellation 22 includes 16 defined constellation coordinates. Again, each coordinate represents a unique combination of carrier magnitude and phase. For example, the magnitude of vector 32 and the phase angle 34 correspond to constellation coordinate 30(a) and the magnitude vector 36 and the phase angle 38 correspond to constellation coordinate 30(b).
It should be appreciated that both constellation 20 and 22 are QAM Square constellations in that a perimeter of an area bounded by the coordinates is square. Constellation 22, representing 2 bits/baud is actually a phase shift keying (PSK) modulation because the amplitude of all coordinates is the same. Similarly, a 3 bits/baud constellation (constellation) is also PSK.
In operation, a transmitter may transmit a modulated carrier with a particular phase and magnitude corresponding to 2/bits baud coordinate 24(a). Due to distortion, the receiver may detect a carrier phase and magnitude corresponding to point 40. The receiver must determine to which of the constellation coordinates 24(a)-24(d) the received point 40 corresponds in order to recover the 2 bits of transmitted data. Using a 2 bits/baud transmission, any received point within shaded area 42 will map to constellation coordinate 24(a). Furthermore, any received points outside the shaded area in the same quadrate will be rounded to map to constellation coordinate 24(a).
However, in a 4 bits/baud transmission, a transmitter may transmit a carrier with a phase and magnitude corresponding to point 30(c) which happens to have the same magnitude and phase as the 2 bits/baud coordinate 24(a). And, because of the same distortion, the receiver detects a carrier phase and magnitude corresponding to point 40. While this distortion was tolerable in a 2 bits/baud transmission, the distortion causes a mis-map in a 4 bits/baud transmission because the received point 40 is within shaded area 44 which will map to coordinate 30(d)xe2x80x94not the originally transmitted coordinate 30(c).
While it is therefore obvious that the lower payload encoding would be more tolerable to distortion, slowing the data rate of all transmissions on a network to overcome the worst distortion has the drawback of reducing network throughput.
Therefore, a recognized solution is to use adaptive payload encoding wherein, based on carrier distortion between a transmitter and a receiver, the maximum payload encoding can be selected which still enables the receiver to properly distinguish between combinations of carrier phase and amplitude. Therefore, if there is relatively little carrier distortion between a particular transmitter and a particular receiver, the two will negotiate a large payload encoding, such at 8 bits per baud for rapid data transmission. However, if there is significant carrier distortion between a particular transmitter and a particular receiver, the two will negotiate a smaller payload encoding, such as 2 bits per baud, to assure error free transmission.
It is desirable that the maximum carrier amplitude is the same regardless of which payload encoding is used, typically, the magnitude of both the I-value and the Q-value of the four outermost constellation coordinates in each of the QAM Square constellations (e.g. 2 bits/baud (PSK), 4 bits/baud, 6 bits/baud, and 8 bits/baud) a value equal to the square root of one half of the maximum amplitude squared and each inner coordinate to a fractional value less than such values. However, an adequate number of bits (for example 18 bits) of precision would be needed to map a symbol in the constellation tables to maintain an adequate signal to noise ratio for reliably recovering the symbol in receiver. Merely reducing the precision by one bit would decrease the signal to noise ratio by 6 db.
The problem is that in such an adaptive environment where payload encoding options include 2, 3, 4, 5, 6, 7, or 8 bits per baud, both the transmitter and the receiver must be able to accommodate all payload options. Such adaptive systems are typically implemented by digital signal processing (DSP) which enables the calculations to be performed quickly enough to transmit and receive the data. The problem is that very large, complex, and costly digital signal processing systems would be required to perform the necessary math within the necessary time window. Moreover, and at least a portion of such digital circuitry needs to be dedicated to each of the possible payload encoding options thereby increasing the total size and cost of the digital circuitry by up to a factor of up to several times that that would be needed for a non adaptive system. Therefore, based on recognized industry goals for size and cost reductions, what is needed is an adaptive slicing device and method for recovering data from a received modulated carrier that do not suffer from the size, cost, and complexity disadvantages of known systems.
A first aspect of the present invention is to provide a network transceiver configured for receiving a complex modulated carrier signal from another network transceiver via a network medium, a receiver portion of a network transceiver comprising: a) an analog to digital converter generating a digital carrier signal representing the complex modulated carrier signal; b) a mixer circuit generating a baseband I-signal and a baseband Q-signal in response to the digital carrier signal; and c) a slicer, detecting and scaling each of the I-signal and the Q-signal, calculating the minimum distance, on a complex decoding constellation, between a received data point, represented by the scaled I-signal and the scaled Q-signal, and one of a plurality of defined constellation coordinates for a particular payload encoding, and generating digital data corresponding to the one of the constellation coordinates.
The I-signal and the Q-signal represent a sequence of received data points and the slicer sequentially operates on each of a sequence of received data points to recover the corresponding data symbols.
The network transceiver may further include a memory storing a plurality of complex decoding constellations wherein each of the plurality of complex decoding constellations represents each of a plurality of payload encoding specifications to which the complex modulated carrier may comply. Each of plurality of constellation coordinates in each of the plurality of complex decoding constellations is represented by an integer I-value and an integer Q-value and is equally spaced from adjacent constellation coordinates in the complex decoding constellation. Equal spacing is accommodated by assuring that each constellation coordinate is located at both an I-coordinate value and a Q-coordinate value that are an odd multiple (e.g. 1, 3, 5, 7, etc) of an integer value
Each of the plurality of complex decoding constellations includes an outermost constellation coordinate and the magnitude of the outermost constellation coordinate in each of the complex decoding constellations is approximately equal to the magnitude of the outermost constellation coordinate in each of the other of the plurality of complex decoding constellations.
In a preferred example, at least a portion of the complex decoding constellations are QAM Square constellations. Further, a portion of the complex decoding constellations may be QAM Cross constellations. The QAM Square constellations may include 2 bits/baud (PSK), 4 bits/baud, 6 bits/baud, and 8 bits/baud and the QAM Cross constellations may include 3 bits/baud (PSK), 5 bits/baud, and 7 bits/baud.
A second aspect of the present invention is to provide a method of receiving a complex modulated carrier signal from another network transceiver via a network medium, the method comprising: a) converting the complex modulated carrier signal to a digital carrier signal; b) generating a baseband I-signal and a base band Q-signal from the digital carrier signal; c) generating a scaled I-signal and a scaled Q-signal by multiplying each of the baseband I-signal and the baseband Q-signal by a scaler; d) sequentially mapping each of a scaled I-signal value and a scaled Q-signal value to one of a plurality of constellation coordinates in a complex decoding constellation; and e) generating digital data corresponding to the one of the constellation coordinates.
The method may further include selecting one of a plurality of complex decoding constellations and the step of sequentially mapping includes sequentially mapping each of a combination of a scaled I-signal value and a scaled Q-signal value to one of a plurality of constellation coordinates in the selected one of the plurality of complex decoding constellations. Each of the plurality of complex decoding constellations may represent each of a plurality of payload encoding specifications to which the complex modulated carrier may comply. Further, each of the plurality of constellation coordinates in each of the plurality of complex decoding constellations is represented by an integer I-value and an integer Q-value and is equally spaced from adjacent constellation coordinates in the complex decoding constellation.
Each of the plurality of complex decoding constellations includes an outermost constellation coordinate comprising a magnitude which is approximately equal to the magnitude of the outermost constellation coordinate in each of the other of the plurality of complex decoding constellations.
In a preferred example, at least a portion of the complex decoding constellations are QAM Square constellations. Further, a portion of the complex decoding constellations may be QAM Cross constellations. The QAM Square constellations may include 2 bits/baud (PSK), 4 bits/baud, 6 bits/baud, and 8 bits/baud and the QAM Cross constellations may include 3 bits/baud (PSK), 5 bits/baud, and 7 bits/baud.
A third aspect of the present invention is to provide a network transceiver configured for receiving a complex modulated carrier signal, modulated in accordance to one of a plurality of payload encoding specifications, from another network transceiver via a network medium, the network transceiver comprising: a) an analog to digital converter generating a digital carrier signal representing the complex modulated carrier signal; b) a mixer circuit generating a baseband I-signal and a baseband Q-signal in response to the digital carrier signal; c) a memory storing a plurality of complex decoding constellations, one constellation corresponding to each one of the plurality of payload encoding specifications, wherein each of the complex decoding constellations includes an outermost constellation coordinate with a magnitude which is approximately equal to that of the outermost constellation coordinate in each of the other of the plurality of complex decoding constellations; and d) a slicer, generating a scaled I-value and a scaled Q-value, sequentially mapping each of a combination of the scaled I-value and the scaled Q-value to one of a plurality of constellation coordinates in one of a plurality of complex decoding constellation, and generating digital data corresponding to the one of the constellation coordinates.
Preferably, each of the plurality of constellation coordinates in each of the plurality of complex decoding constellations is equally spaced from adjacent constellation coordinates in the complex decoding constellation and each is represented by an integer I-value and an integer Q-value.
In a preferred example, at least a portion of the complex decoding constellations are QAM Square constellations. Further, a portion of the complex decoding constellations may be QAM Cross constellations. The QAM Square constellations may include 2 bits/baud (PSK), 4 bits/baud, 6 bits/baud, and 8 bits/baud and the QAM Cross constellations may include 3 bits/baud (PSK), 5 bits/baud, and 7 bits/baud.